Downlights are lighting fixtures mounted in a ceiling for illumination directly below them. These ubiquitous luminaires generally comprise an incandescent spotlight mounted within a can. The can is typically closed except at the bottom, so any hot air becomes trapped within the can. Even in the rare cases when heat is transmitted through the can to a heat sink or heat exchanger on the outside of the can, the heat exchanger is typically in stagnant air within a false ceiling, and is not very effective. In most cases, not only is there no heat exchanger, the can is actually insulated to prevent heat from being delivered into the space within the false ceiling. Since incandescent bulbs operate hot anyway, they are not thermally bothered by the can being a trap for hot air. It would be highly desirable to replace the light bulbs with lamps using light-emitting diodes (LEDs), which are more efficient. A white LED system, using blue LEDs combined with yellow phosphor, would be suitable.
LEDs, however, are sensitive to excessive temperatures and thus find downlights to be a more difficult lighting application than anticipated. This is because their heat cannot safely be dissipated passively into the stagnant hot air of the typical downlight can. This typically limits the total wattage that can be handled in a solid state LED downlight to a maximum power of approximately 4 Watts. This limit can only be overcome if the can is dramatically widened to aid in cooling for the sake of heat management, a severe limitation on the situations in which the LED downlight can be used. Furthermore, the best commercially available 4 Watt LED sources have an efficacy of 60 lumens per Watt including driver losses. This limits the solid state downlight to a flux of only approximately 250 lumens. A flux output of 600 to 1000 lumens is desirable for a downlight, and it is desirable for the downlight to be able to operate in a standard size, typically 4″-6″ (10 to 15 cm) diameter ceiling can. This is achievable for an LED or comparable solid-state downlight if the heat management can handle a minimum of 10 Watts.